The present invention relates to an optical information recording medium which is capable of recording and reading back information.
Various principles are known which are used for recording information on a thin layer (i.e., recording layer) by irradiation of laser light. Among these principles, those which make use of a phase transition (also called "phase change") of layer materials or of a change of atomic arrangement caused by irradiation of laser light (e.g., photo-darkening) have an advantage in that they can give an information recording medium of double-sided disk structure by directly laminating two disk members because they are accompanied by little deformation of the thin layers. Furthermore, recording layers of GeSbTe or similar type have the advantage of permitting the rewriting of information.
In a recording layer of this kind, however, when the layer is subjected to a large number (exceeding 10.sup.4) of times of rewriting by means of a sample servo system or a mark edge recording, which are conducted for attaining a high density recording, the recording layer thickness changes as the result of a flow of the recording layer and the wave form of the reproduced signal undergoes distortion. The cause of the flow of the recording layer is presumably that the recording layer becomes flowable by the laser beam irradiation at the time of recording and the recording layer is pushed little by little as the result of deformation of the protective layer and the intermediate layer caused by thermal expansion.
For example, reference 1 (T. Ohta et al., "Optical data Storage", '89 Proc. SPIE, 1078, 27 (1989)) discloses a method of preventing the flow of the recording layer by decreasing the thickness of the recording layer to reduce its heat capacity and making use of the resultant increase in the effect of the adhesive force of the layer with adjacent layers. Reference 2 (Hirotsune, Terao, Miyauchi, Minemura and Fushimi, Collection of preliminary papers for 41th applied physics joint lecture meeting, p. 1000) discloses a method of preventing the flow of the recording layer by adding a high melting point ingredient. These methods can suppress the flow of the recording layer to a certain extent. However, when a large, number of times of rewriting is repeated, the fluctuation of reflectivity level takes place.
To improve the overwrite jitter characteristic in mark edge recording, a disk increased in transmitted light intensity has been proposed in reference 3 (Okubo, Murahata, Ide, Okada and Iwanaga, Collection of preliminary papers for 5th phase change recording symposium, p. 98). The disk has a structure of PC substrate/ZnS--SiO.sub.2 (250 nm)/Ge.sub.2 Sb.sub.2 Te.sub.5 (15 mm)/ZnS--SiO.sub.2 (18 nm)/Si (65 nm).
On the other hand, in optical disks obtained by replicating, as surface unevenness on the substrate surface, analog information signals produced by the FM modulation of image signals and sound signals and digital information signals, such as electronic computer data, facsimile signals and digital audio signals, and in optical disks having information recording thin layers or the like which are capable of recording signals and data at the real time by means of recording beams, such as laser beams and electron beams.
The signal readback resolution is almost entirely determined by the wavelength .lambda. of the light source of the reproduction optical system and the numerical aperture NA of the objective lens, the recording mark period 2NA/.lambda. representing the reading limit.
As to the means for obtaining a high density recording, a method in which data recorded by ruggedness are read back by using a medium which changes its reflectivity by phase change and the medium used therein are described in reference 4 (K. Yasuda, M. Ono, K. Aritani, A. Fukumoto, M. Kaneko; Jpn, J. Appl. Phys. vol. 32 (1993), p. 5210). In this method, also, the layer for ultra-high resolution reading undergoes flowing of the layer and gives rise to fluctuation of reflectivity level when subjected to a very large number, exceeding 10.sup.4, of times of reading, so that the possible number of times of reading is restricted.
In the present specification, the term "phase change" is used not only to indicate the phase change between the crystalline and the amorphous states but also to imply the phase change of melting (i.e., change into liquid phase) and recrystallization and the phase change between the crystalline and another crystalline states.